In semiconductor manufacturing processes, lots of manufacturing or inspection processes are performed in a vacuum environment with a pressure much lower than atmospheric pressure. In order to maintain the low pressure environment in the main processing chamber, the design concept of using a transfer chamber is employed. The transfer chamber is used to transfer a wafer or wafers into the processing or specimen chamber without damaging the low pressure condition in the vacuum environments.
Without limiting the scope of the present invention, the process of gas vent in a transfer chamber is illustrated with a SEM (scanning electron microscope) system. Referring to FIG. 1, a SEM specimen chamber (SC) 10 with accompanying load chamber (LC) 12 and transfer chamber (XC) 14 is illustrated. Since the SEM inspection is performed with the generation of an electron beam, a vacuum environment is needed in the specimen chamber 10 to provide low pressure. A wafer can be loaded into the specimen chamber 10 from the environment with the atmospheric pressure without influencing the vacuum condition of the specimen chamber design.
In general, the wafer to be inspected is loaded into the transfer chamber 14 at one atmospheric pressure (1 atm). The lead or cover 14a of the transfer chamber 14 is closed and the pressure is low down to the specified pressure of the vacuum requirements. The pressure can be drawn down by a well-known pump system. The wafer is then transferred into the specimen chamber 10 through the transformation of the load chamber 12. Thus the vacuum environment in the specimen chamber 10 can be maintained by the pressure regulation in the transfer chamber 14. After the SEM inspection process of the wafer in finished, the wafer is transferred back into the transfer chamber 14 through the load chamber 12. The pressure in the transfer chamber 14 is raised up by a gas vent system 16 which supplies gas to increase the pressure in the transfer chamber to one atmospheric pressure (1 atm).
During the gas vent process in the transfer chamber 14, the gas flow rate and cleanness of the supplying gas must be carefully controlled. In general, the pressure in the transfer chamber is very low. As an example, the pressure can be as low as about 100 mTorr (mili-Torr) to about 20 mTorr for the SEM system. In such a low pressure, any abrupt pressure change during the vent process can induce a strong gas flow and air turbulence. The gas flow causes a serious problem of particle redistribution and the wafer suffers from the particle contamination problem after the vent process.
In the traditional design of the gas vent system 16, only a set of valves are used to control the gas flow. Thus the vent process with a high pressure difference and gas flow-induced friction distributes lots of undesired particles to the wafer. In the conventional vent process, the wafer is found to have several hundred particles added after the process. The particle contamination problem can influence the yield of the process and even damage the devices on the wafer. For performing an inspection process free of the particle contamination problem, a modified or redesigned vent system is highly needed.